During April 16-18, 2007, central and southeastern New Hampshire experienced severe flooding as a result of up to 7 inches of rainfall from a storm that stalled off the New England coast. As a result of the flooding, a Presidential Disaster Declaration was issued on April 27, 2007. On that day, disaster declarations were announced for Grafton, Hillsborough, Merrimack, Rockingham, and Strafford Counties. On May 10, 2007, Belknap County was added to the disaster declaration.
Following the flooding, the U.S. Geological Survey, in a cooperative investigation with the Federal Emergency Management Agency, determined the peak stages, peak discharges, and recurrence-interval estimates of the April 2007 flood at 57 streamgages and 4 ungaged sites in and adjacent to the counties named in the disaster declaration. Data from flood-insurance studies published by the Federal Emergency Management Agency also were compiled for each streamgage site for comparison purposes.
The peak discharges during the April 2007 flood were the highest ever recorded at five long-term (more than 10 years of record) streamgage sites on the New Hampshire-Salmon Falls River at Milton, Cocheco River near Rochester, Oyster River near Durham, Contoocook River at Peterborough, and South Branch Piscataquog River near Goffstown. In addition, peak discharges equaled or exceeded a 100-year recurrence interval at 10 streamgages and a 50-year recurrence interval at 16 streamgages. The most severe flooding occurred in Rockingham, Strafford, Merrimack, and Hillsborough Counties.